1. Field of the Invention
This invention relates to techniques and devices for separating plasma from blood by filtration and is particularly directed to filtration at low pressures.
2. Description of the Background
Many diagnostics are carried out in the clinical field utilizing blood as a sample. Although some of these techniques can be carried out on whole blood, it is necessary in many instances to utilize serum or plasma as the sample in order to obtain an accurate reading. For example, red blood cells (erythrocytes) scatter and absorb light and could adversely affect a measurement of either reflected or transmitted light of a diagnostic test relying on either of these measurement techniques.
Traditionally, plasma and serum have been separated from whole blood by centrifuging either before (for plasma) or after (for serum) clotting. However, centrifugation is time consuming and requires equipment that is not generally available outside the clinical laboratory. Accordingly, field testing of numerous blood substances that require serum or plasma is difficult.
A number of techniques have been devised to avoid this problem. The techniques generally utilize a filtering device capable of separating red blood cells from plasma. Numerous materials have been used in the past to form filters. Paper, non-woven fabric, sheet-like filter material composed of powders or fibers such as man-made fibers or glass fibers, and membrane filters having suitable pore sizes have been proposed. For example, U.S. Pat. No. 4,256,693 to Kondo et al. discloses a number of filter materials in a multi-layered integral chemical analysis element for use with blood. U.S. Pat. No. 4,477,575 to Vogel et al. describes a composition and process for permitting the separation of plasma or serum from whole blood utilizing glass fibers in combination with other absorbent layers.
However, these prior art techniques have proven to be unsuitable for use in applications which, because of space and volume restraints, can only utilize a small filter in a device in which a single drop of blood is separated and the plasma is transported through the device solely by means of capillary action. Accordingly, further refinement in blood separation techniques is desirable.